The present invention relates generally to radio communication systems using the time division duplexing (TDD) mode. More particularly, the invention relates to assignment of slots and slot-to-cell assignments in radio communication systems using TDD.
Cellular systems generally divide the time axis into intervals of equal durations called frames. Cellular systems employing the TDD scheme divide frames into a finite number of intervals of equal duration, called slots, and allow a cell to use some or all of the slots for uplink transmissions (mobile-to-base) or downlink (base-to-mobile) transmissions. The slot assignment of a cell defines how each slot is used by this cell. There are three possible ways for a cell to use a slot: uplink transmissions; 2) downlink transmissions; or 3) the slot is not used.
The slot assignment of a cell can be varied by the system in order to adapt to the long-term variations of the traffic load. For example, the system may modify the assignment of one slot from uplink to downlink if the intensity of downlink traffic increases while the uplink traffic decreases. In addition, different cells of a system do not generally need to have the same slot assignment. Accordingly, if traffic characteristics in one geographical area are different from another area, the cells covering those areas may have different slot assignments to best adapt to local traffic conditions.
In the prior art, a simple approach in order to avoid base-to-base and mobile-to-mobile interference is to use the same slot assignment for all cells in the same geographic area; only allowing different assignments between cells that are clearly isolated from each other. The obvious disadvantage of this approach is when cells are deployed in a way to provide continuous coverage, as is often the case, it is difficult to segregate one subset of cells from another, unless the use of certain slots are completely disallowed in some cells. This ultimately results in a capacity loss of the system.
Therefore, both mobile-to-mobile and base-to-base interference restrict the use of independent slot assignments between cells in the same geographical area. What is needed is a system which avoids the loss of capacity as the traffic asymmetry metric varies over a coverage area.